Acoustic duct speaker system

ABSTRACT

A bass reflex cabinet speaker assembly, e.g. as for use with stereo and high-fidelity speaker systems, having a port coupling the cabinet to the air, and having mounted within said port an acoustic duct formed of a large number of contiguous conduits each of a different length, differing progressively from the center to the periphery of the duct and preferably so formed that the end of the duct is in a concave or convex parabolic shape. In the most advantageous forms of the invention, a flexible corrugated paperboard sheet-or other sheet having a series of conduits therein-may be cut generally obliquely either rectilinearly or curvilinearly and rolled up to form the acoustical duct.

United States- Patent Hopkins [54] ACOUSTIC DUCT SPEAKER SYSTEM [72]Inventor: Herbert J. Hopkins, 24 Portland St.,

Old Orchard Beach, Maine 04064 [22] Filed: Oct. 8, 1970 [21] Appl.No.:79,143

[151 3,684,051 [451 Aug. 15,1972

423,291 7/l947 Italy ..l8l/3l B Pn'mary Examiner-Stephen J. TornskyAttorney-Pearson & Pearson [57] ABSTRACT A bass reflex cabinet speakerassembly, eg as for use with stereo and high-fidelity speaker systems,having a 'port coupling the cabinet to the air, and having mountedwithin said port an acoustic duct formed of a large number ofcontiguousconduits each of a different length, differing progressivelyfrom the center to the periphery of the duct and preferably so formedthat the end of the duct is in a concave or convex parabolic shape. Inthe most advantageous forms of the invention, a flexible corrugatedpaperboard sheet- -or other sheet having a series of conduits therein--may be cut generally obliquely either rectilinearly or curvilinearlyand rolled up to form the acoustical duct.

14 Claims, 7 Drawing Figures PATENTED AUG 1 5 I972 FIG. 2

HERBERT J. HOPKINS lm/en for By Pea/14m "7 1 ACOUSTIC DUCT SPEAKERSYSTEM BACKGROUND OF THE INVENTION A considerable effort has been madeto develop improved speaker systems for use in stereo and highfidelitysound systems. The primary object of this effort has been to obtain thebest possible sound output at the lowest possible cost to themanufacturer and consumer.

It is already known to achieve acoustical phase inversion by way of aport coupling the interior of a low frequency speaker cabinet to theenvironmental air. In such speaker systems, called bass-reflex cabinets,the effective stiffness of air within the cabinet is resonated with theeffective mass of air associated therewith through the port. However, ithas remained a problem to provide a suitable means for moderating of theair through the port, say by use of an acoustical duct, which means willallow the system to give acceptable tone'reproduction and which means is(l) relatively non-critical with respect to. the required precision ofthe turning, and (2) relatively non-critical with respect to theparticular dimensions of the speaker box in which it is located.

There have been a number of attempts to improve loudspeaker systems byproviding interior bafiling within a loudspeaker cabinet. Most suchsystems, if really effective, are undesirably expensive. Witness, forexample, the loudspeaker systems disclosed in U.S. Pat. No. 2,880,817 toH. S. Burns et al. wherein the interior of the loudspeaker box isdivided into a plurality of passages by means of a rather complexconstruction technique.

SUMMARY OF THE INVENTION Therefore, it is an object of the invention toprovide an improved bass reflex loudspeaker system comprising animproved air coupling means.

It is a further object of the invention to provide improved acousticaltubes for use in such a bass reflex loudspeaker.

Another object of the invention is to provide a novel process for makingan acoustical duct having a very large number of tubes or passages eachof a different length and, consequently, each of a different resonantfrequency.

Other objects of the invention will be obvious to those skilled in theart on their reading of the instant application.

The above objects have been substantially achieved by mounting animproved acoustical duct in a port of a bass reflex box. This acousticalduct is advantageously formed of a single, flexible, sheet materialhaving a row of parallel flutes, tubes or conduits, which may be rolledup to form a duct. A corrugated paperboard sheet would be one example ofsuch a material. When the sheet of pre-selected length is cut at apre-selected angle, or curvilinear line, the desired distribution of anyconduit lengths is readily achieved.

The flexible sheet can be formed of a number of other materials, suchas, for example, honeycombed material or ribbed or a sinusoidally-shapedmaterial which winds upon itself in such a way as to provide a pluralityof conduits through a roll formed thereof. It has been found that use ofmultiple tubes of different lengths not only results in a broaderresonant band but also minimizes the effect of self-resonance developedwithin the duct structure.

. It should be noted that a loudspeaker can comprise one or more of theacoustic ducts of the invention. Indeed, a single duct can beconstructed to cover different resonant ranges- -by, for example,forming a core section of a duct having conduits of varying length andof one diameter and forming an outer section of the same duct from asheet material having conduits also of varying length but ofsignificantly larger or smaller diameters. Instead of varying diametersof the individual conduits, it is possible to form compound ducts havingtubes of two or more distinctly different length ranges.

In the most advantageous embodiments of the invention, the conduitsformed in the acoustic tube are from about three quarters of an inch toabout twelve inches in length and from about two inches to about teninches in diameter. The total number of conduits are conveniently fromone hundred to four hundred conduits per acoustic tube.

ILLUSTRATIVE EMBODIMENT OF THE INVENTION In this application andaccompanying drawings there is shown and described a preferredembodiment of the invention and there are suggested various alternativesand modifications thereof, but it is to be understood that these are notintended to be exhaustive and that other changes and modifications canbe made within the scope of the invention. These suggestions herein areselected and included for purposes of illustration in order that othersskilled in the art will more fully understand the invention and theprinciples thereof and will be able to modify it and embody it in avariety of forms, each as may be best suited in the condition of aparticular case.

IN THE DRAWINGS FIG. 1 is a front elevation of a typical bass reflexloudspeaker cabinet with my acoustic duct mounted therein;

FIG. 2 is a side elevation thereof in section on line 2-2;

FIGS. 3 and 4 each show an acoustic duct prepared from rolled corrugatedpaperboard;

FIG. 5 shows an acoustic duct formed of a plurality of tubes ofdiffering lengths;

FIG. 6 shows a corrugated paperboard piece before it is rolled up toform the acoustic duct shown in FIG. 3; and

FIG. 7 shows a section of flexible corrugated sheet material which canbe used to form an acoustic duct ac cording to the invention.

DESCRIPTION OF PREFERRED EMBODIMENT cal, about 3% inches in diameter andabout 8 inches long, high frequency speaker 14 is about 4 inches indiameter and low frequency speaker 16 is about 15 inches in diameter.The cabinet, or box 10, itself is 21 inches wide by 29 inches high and16% inches deep.

FIG. 7 shows a piece 20 of single-sided corrugated paperboard, amaterial suitable for winding into a duct flutes 28 become pressedagainst the adjacent convolution of liner 26.

FIG. 6 illustrates a typical blank 27 cut generally 10 obliquely from aflat rectangular sheet and at such an angle, or simple, or compoundcurve as to be useful in pre-forming the corrugated paperboard before itis wound into an acoustical duct 30 as seen in FIG. 3 (or an acousticalduct 32 as illustrated in FIG. 4), depending on the direction in whichthe corrugated paperboard 20 is wound into a coil, and depending onwhether the high side 23 or the low side 25 is at the center of thecoil.

As shown in FIG. 6, the generally oblique cut line 29 preferablyterminates proximate the outer end of the duct in a terminal portion 31,parallel to the base, thereby forming a cuff 33 (FIG. 4) of uniformlength which encircles the concave or convex end, lends strength andpermits the parabolic end to be adhered in port 42 if desired.

In each of the acoustic ducts of FIGS. 3, 4 or 5, the close packed mass38 of elongated tubes 22 includes a flat, planar end 40, filling thesound balancing port 42 in cabinet 10. The tubes are in parallelism, andof uniform cross section from end to end, and the mass terminates inpreferably the exterior parabolic end face 44 as in FIG. 3 or theinterior parabolic end face 46 of FIG. 4. The mass 38 of individualtubes 36 of FIG. 5 is not satisfactory when the mass is cut at an angle39 because-some of the tubes 36 will be of equal length and equalresonance. Therefore it is preferred that each tube 36 be out while inthe flat configuration of FIG. 6 and then rolled and adhered into a massof tubes such as 38.

' 'It will be understood that the acoustic ducts of the invention mayhave both ends concave, convex and parabolic so long as the individualtubes, or conduits, are all of different lengths.

In FIG. 5, the acoustical duct 34 is preferably formed of thin-walledpolymer tubes 36 fastened together by adhesives.

It is to be understood that the following claims are intended to coverall of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention'which mightbe said to fall therebetween.

What is claimed is:

1. In a loudspeaker cabinet of the bass-reflex type, the improvementwherein an acoustic duct mounted in a wall thereof, and thereby forminga conduit between the interior of the loudspeaker cabinet and thesurrounding air, is formed of a large number of conduits of differinglengths, all said conduits being formed into an integral acoustic ductof generally cylindrical shape said conduits being of progressivelydifferent lengths between the center and the circumference thereof.

2. A loudspeaker as defined in claim 1 wherein said acoustic duct isformed of at least one continuous flexible sheet so cut that the ends ofthe conduits terminating within the loudspeaker cabinet all terminate atdifferent distances from the wall of said loudspeaker cabinet.

3. A loudspeaker as defined in claim 2 wherein the flexible sheet is socut that the ends of the conduits terminating within the loudspeakercabinet form a parabola-shaped end on said duct.

4. A loudspeaker as defined in claim 1 wherein said large, number ofconduits of differing lengths are formed by cutting a flexiblecorrugated sheet at an angle and rolling said sheet up to form acylinder.

5. A loudspeaker as defined in claim 4 wherein said flexible corrugatedsheet is corrugated paperboard.

6. In combination with a loudspeaker cabinet of the bass reflex type,

a generally cylindrical acoustic duct mounted in a wall of said cabinet,said duct comprising a large number of elongated, small diameterconduits all of differing lengths, said conduits being from about threequarters of an inch to about 12 inches in length, said duct being fromabout 2 inches to about 10 inches in diameter and said conduits being ofprogressively different lengths from the center to the periphery of saidcylindrical acoustic duct.

7. An acoustic duct as defined in claim 6 wherein said ducts are formedof a sheet of corrugated material cut generally obliquely and rolled upupon itself.

8. An acoustic duct as defined in claim 6 wherein said ducts are formedof corrugated paperboard.

9. An acoustic duct as defined in claim 6 wherein said cylindrical ductis formed by adhering together at least about two hundred tubes to formsaid conduit,

each tube being of a different length but of identical whereby a broaderresonant band results which minimizes self resonance within said duct.11. An acoustic duct as specified in claim 10, wherein:

each said tube is of uniform cross section from one end to the otherthereof. 12. An acoustic duct as specified in claim 10, wherein:

said tubes are formed by the corrugated flutes and liner of a corrugatedpaperboard-like sheet material rolled into a helical configuration. 13.An acoustic duct as specified in claim 10, wherein:

the said one end of said duct is flat and planar, said tubes are inparallelism, and the said other end of said duct defines a parabolicconfiguration.

14. An acoustic duct as specified in claim 10 wherein:

said duct is of predetermined length and diameter to tune the cabinet toa desired frequency.

i R I

1. In a loudspeaker cabinet of the bass-reflex type, the improvementwherein an acoustic duct mounted in a wall thereof, and thereby forminga conduit between the interior of the loudspeaker cabinet and thesurrounding air, is formed of a large number of conduits of differinglengths, all said conduits being formed into an integral acoustic ductof generally cylindrical shape said conduits being of progressivelydifferent lengths between the center and the circumference thereof.
 2. Aloudspeaker as defined in claim 1 wherein said acoustic duct is formedof at least one continuous flexible sheet so cut that the ends of theconduits terminating within the loudspeaker cabinet all terminate atdifferent distances from the wall of said loudspeaker cabinet.
 3. Aloudspeaker as defined in claim 2 wherein the flexible sheet is so cutthat the ends of the conduits terminating within the loudspeaker cabinetform a parabola-shaped end on said duct.
 4. A loudspeaker as defined inclaim 1 wherein said large number of conduits of differing lengths areformed by cutting a flexible corrugated sheet at an angle and rollingsaid sheet up to form a cylinder.
 5. A loudspeaker as defined in claim 4wherein said flexible corrugated sheet is corrugated paperboard.
 6. Incombination with a loudspeaker cabinet of the bass reflex type, agenerally cylindrical acoustic duct mounted in a wall of said cabinet,said duct comprising a large number of elongated, small diameterconduits all of differing lengths, said conduits being from about threequarters of an inch to about 12 inches in length, said duct being fromabout 2 inches to about 10 inches in diameter and said conduits being ofprogressively different lengths from the center to the periphery of saidcylindrical acoustic duct.
 7. An acoustic duct as defined in claim 6wherein said ducts are formed of a sheet of corrugated material cutgenerally obliquely and rolled up upon itself.
 8. An acoustic duct asdefined in claim 6 wherein said ducts are formed of corrugatedpaperboard.
 9. An acoustic duCt as defined in claim 6 wherein saidcylindrical duct is formed by adhering together at least about twohundred tubes to form said conduit, each tube being of a differentlength but of identical diameter.
 10. In a low frequency loudspeakercabinet having a port for balancing sound to improve reception: a closepacked mass of elongated tubes, jointly defining an acoustic duct, saidduct having one end mounted in said port for passing sound from insidesaid cabinet through said tubes to outside said cabinet; each said tubebeing of a different length, and having a different resonant frequencyfrom the remaining said tubes in said mass; said tubes varyingprogressively in length from the center to the periphery of said mass,whereby a broader resonant band results which minimizes self resonancewithin said duct.
 11. An acoustic duct as specified in claim 10,wherein: each said tube is of uniform cross section from one end to theother thereof.
 12. An acoustic duct as specified in claim 10, wherein:said tubes are formed by the corrugated flutes and liner of a corrugatedpaperboard-like sheet material rolled into a helical configuration. 13.An acoustic duct as specified in claim 10, wherein: the said one end ofsaid duct is flat and planar, said tubes are in parallelism, and thesaid other end of said duct defines a parabolic configuration.
 14. Anacoustic duct as specified in claim 10 wherein: said duct is ofpredetermined length and diameter to tune the cabinet to a desiredfrequency.